An imaging apparatus such as a printer, a facsimile machine, a plotter, or a printer/fax/copier multifunction machine may correspond to a serial imaging apparatus that includes a carriage having a recording head that is made up of one or more liquid discharge heads configured to discharge droplets of recording liquid (e.g., ink), for example. The serial imaging apparatus is configured to serially scan the carriage in a direction perpendicular to a conveying direction of a recording medium (also referred to as ‘paper’ hereinafter although the recording medium is not limited to paper and may be made of some other suitable material), and intermittently convey the recording medium according to a recording width. In other words, the serial imaging apparatus is configured to alternate between conveying the recording medium and recording an image on the recording medium to realize imaging (also referred to recording or printing).
When bidirectional printing that involves forming an image in two different directions, namely an outward direction and a homeward direction, is realized by the carriage of the serial imaging apparatus, the order in which colors are discharged is reversed between the case of recording in the outward direction and the case of recording in the homeward direction, and color density unevenness may occur in bands (bidirectional color difference) as a result of the difference in the color discharging order. In this respect, various measures are implemented in order to reduce such bidirectional color difference.
As a first example, Japanese Laid-Open Patent Publication No. 2001-205828 discloses an apparatus having two sets of heads that are arranged to be symmetric to each other with respect to a scanning direction, each set of heads being configured to discharge cyan (C), magenta (M), and yellow (Y) inks to form a relatively large dot or a relatively small dot, the apparatus being characterized by altering the ink discharging order (e.g., C→M and M→C) in forming plural secondary color level pixels arranged in the raster direction so that there will be no difference in the discharging order depending on whether scanning is performed in the outward direction or homeward direction and color unevenness caused by the difference in the discharging order may be reduced. It is noted that similar disclosures are also made in Japanese Laid-Open Patent Publication No. 2002-137421, Japanese Laid-Open Patent Publication No. 2001-96770, Japanese Laid-Open Patent Publication No. 2001-96771, and Japanese Laid-Open Patent Publication No. 2001-130033.
As a second example, Japanese Laid-Open Patent Publication No. 8-295034 discloses an imaging apparatus including a first head having discharge heads respectively for cyan, magenta, and yellow arranged in this order, and a second head having discharge heads for the above colors arranged in a reverse order, which first and second heads are arranged to be symmetric to each other with respect to the main scanning direction with an array of black discharge heads disposed in between the first and second heads, the first head being used to discharge ink upon scanning in the outward direction, and the second head being used to discharge ink upon scanning in the homeward direction.
As a third example, Japanese Laid-Open Patent Publication No. 2000-79681 discloses an imaging apparatus including a first head that has discharge units for discharging the different types of inks used in the apparatus that are arranged in the recording medium conveying direction and are configured to acquire image characteristics information pertaining to image characteristics of input image information; and a second head that has discharge units identical to those of the first head that are arranged to be symmetric to the discharge units of the first head; wherein recording dots based on the input image information are assigned to the first head and the second head based on the image characteristic information by a multi-path data generating unit; multi-path recording of the first head is controlled by a first head control unit according to first recording dots; and multi-path recording of the second head is controlled by a second head control unit according to second recording dots.
As a fourth example, Japanese Laid-Open Patent Publication No. 2001-113736 discloses an inkjet head that is used in a serial scanning inkjet recording apparatus in which inkjet head plural nozzle arrays each corresponding to ink that can be discharged are arranged in parallel with respect to the scanning direction, wherein at least three nozzle arrays for discharging at least two types of ink are aligned in the same order with respect to the outward scanning direction and the homeward scanning direction.
As a fifth example, Japanese Laid-Open Patent Publication No. 2001-171119 discloses a liquid discharge head that discharges a first liquid and a second liquid that is of a different type from the first liquid from differing discharge outlets to realize recording, the liquid discharge head including a first discharge outlet array group made up of plural discharge outlet arrays each having plural discharge outlets arranged at a predetermined pitch in a direction that is different from the scanning direction which arrays are arranged such that corresponding discharge outlets of the discharge outlet arrays are brought in line with respect to the scanning direction; and a second discharge outlet array group made up of discharge outlets having a similar configuration to that of the first discharge outlet array group that is arranged next to the first discharge outlet array group; wherein the first discharge outlet array group includes a first discharge outlet array for discharging a first liquid and a second discharge outlet array for discharging a second liquid; the second discharge array group includes a third discharge outlet array for discharging the first liquid and a fourth discharge outlet array for discharging the second liquid; and the first discharge outlet array group and the second discharge outlet array group are arranged such that the first discharge outlet array and the third discharge outlet array are adjacent to each other, and the discharge outlets forming the first discharge outlet array and the discharge outlets forming the third discharge outlet array are shifted in their alignment direction so that they complement each other with respect to the scanning direction.
As a sixth example, Japanese Laid-Open Patent Publication No. 7-112534 discloses an imaging apparatus including means for conveying a recording medium and a carriage having ink tanks and a head that outputs input image information onto the recording medium by discharging ink from plural nozzles, the nozzles being divided into at least two nozzle groups that are connected to different ink tanks.
As a seventh example, Japanese Laid-Open Patent Publication No. 2003-266749 discloses an imaging apparatus that uses gamma correction to reduce color phase variations caused by the difference in the color discharging order between outward direction recording and homeward direction recording of a print head.
However, in the first example where two sets of heads that discharge dots of differing sizes are symmetrically arranged and the ink discharging order is altered to realize recording in the same ink discharging order for the outward direction and the homeward direction, and in the second through fourth examples where differing heads or nozzle arrays are used for the outward direction and the homeward direction, even when the ink discharging order may be the same, color differences may occur between recording in the outward direction and the homeward direction due to variations in the amount of ink discharged by the different heads.
In the fifth example where a liquid discharge head is used that includes first and second discharge outlet array groups each having plural discharge outlet arrays that are arranged such that corresponding discharge outlets of the discharge outlet arrays are brought in line with respect to the scanning direction, and the pitches of the first discharge outlet array group and the second discharge outlet array group are shifted from each other with respect to a direction perpendicular to the scanning direction, the head structure becomes complicated to thereby result in an increase in manufacturing costs.
In the sixth example where plural nozzles of a head are divided into two nozzle groups that are connected to different ink tanks, two mechanisms for supplying ink have to be provided so that the head structure becomes complicated and large, and the structure of a mechanism for positioning the head may also be complicated and large. Further, there may be an increase in the amount of discarded ink that is discharged for preventing the mixing of color ink and black ink upon simultaneously performing maintenance of the nozzles for the color ink and the black ink.
In the seventh example where gamma correction is used to reduce color phase variations caused by the difference in the ink discharging order between recording in the outward direction and the homeward direction, the gamma correction process may be complicated.